Leakage current sensor for suspension type insulator

ABSTRACT

A sensor apparatus for detecting leakage current in a suspension-type insulator of an electrical power system includes: a housing having a first half of a leakage current sensor contained therein; a door pivotally connected to the housing by a hinge, and having a second half of the leakage current sensor contained therein, the door pivotable about the hinge between an open position in which the first and second halves of the leakage current sensor are separate from each other, and a closed position in which mating surfaces of the first and second halves of the leakage current sensor join together to define a closed perimeter; and a clamping mechanism connected to the housing comprising a first jaw and a second jaw, wherein the jaws are moveable between an open position and a closed position.

BACKGROUND OF THE INVENTION

The present invention relates generally to the monitoring of leakage currents in a transmission system and, more particularly, to a sensor for accurately detecting and monitoring leakage current in suspension type insulators.

In many countries, including the US, wood is utilized as part of the line insulation to improve the basic insulation level (BIL) of the line, as it has been recognized that the lighting performance of transmission lines can be improved by utilizing the wood support or pole. This has, however, not been without problems as there were, and still are, many instances of fires of the wood poles and cross arms caused by low frequency leakage current and sparking on the wood from sources such as leakage current due to insulator contamination despite mitigation measures being taken.

Insulators installed on transmission and distribution systems are exposed to contamination, for example from marine salt, road salt, and industrial pollutants. This contamination can result in flashover of the insulator, usually under light wetting conditions, e.g. condensation, when the salts and water mix to become a conductive electrolyte. Flashover is a problem in that it results in an outage which interrupts power to a utility company's customers.

When the salts on the surfaces of an insulator become wet they form an electrolyte which is conductive. Since the one end of the insulator is energized, and the other end is grounded, currents flow along the insulator surfaces. If these currents are large enough, arcing will occur (called dry band arcing). This arcing will either extinguish or grow to result in a flashover. The magnitude of leakage currents that occur under dry band arcing conditions provide an indication of the risk of the insulator flashing over. If insulators are at risk of flashover due to contamination build-up, utilities can wash the insulators or take other measures, such as redirect power to other transmission and distribution assets.

In addition when composite (also called polymer or non-ceramic) or coated insulators are used the leakage currents and arcing on the surface can degrade the rubber material. By monitoring the leakage currents an indication can be obtained as to the level of degradation.

Some commonly used mitigation measures are listed below; however, none of these mitigation measures provide a means for monitoring and pinpointing potential leakage current problems so that a utility company can take preventative measures.

1. Wrapping metal bands around the wood pole and connecting it to the insulator hardware. This method has the advantage that the reduction in the “insulated” wood path lengths (used as improvement for the BIL) is limited. The conductor material used for this purpose should be compatible with other hardware not to cause corrosion.

2. A small guard electrode, in the form of a coach screw or a multi-spiked plate (e.g. gang-nail), is fastened to the wood outside the rain shadow area and bonded to the insulator hardware. This method has a minimal effect on the BIL of the structure.

3. Application of conducting paint to cover the high resistance zones around metal to wood interfaces. This method has a minimal effect on the BIL of the structure.

4. Bonding of the insulator hardware together with a conductor. The intension with this bond wire is to “balance” the leakage current so that only a small residual current will flow in the pole. There are two variants to this scheme:

-   -   (a) The insulator bases are connected together but not grounded.         The ground lead terminates some distance away to realize the         required BIL phase-to-ground for induced lightning surges.     -   (b) The insulator bases can be bonded together and connected to         ground. In this case the wood is not utilized anymore as part of         the line insulation against lightning. Also here it is important         that the material of the bond wire is selected to be compatible         with the other hardware used to prevent corrosion.

5. An extension of the previous method is to use steel cross-arms to bond the insulator bases together. The steel cross-arm can either be grounded or be left floating depending on whether or not the utility wants wood as part of the line insulation for lightning performance.

6. Finally the insulators used can be upgraded to those with an improved contamination performance. For example porcelain insulators can be replaced by hydrophobic silicone rubber composite insulators. Other options include regular insulator cleaning or the application of silicone grease to insulators.

BRIEF SUMMARY OF THE INVENTION

These and other shortcomings of the prior art are addressed by a leakage current monitoring system that continuously monitors and reports potential leakage current issues to allow an action to be taken to mitigate any potential problems that may arise from the leakage current.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter that is regarded as the invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:

FIG. 1 is a schematic side view of a portion of a utility pole carrying a suspension-type insulator and a sensor unit constructed according to an aspect of the present invention;

FIG. 2 is a perspective view of the sensor unit shown in FIG. 1;

FIG. 3 is another view of the sensor unit shown in FIG. 1;

FIG. 4 is a view of the sensor unit of FIG. 1 with a cover removed to show the internal components;

FIG. 5 shows a door of the sensor unit of FIG. 1; and

FIG. 6 is a block diagram showing the operation of the sensor unit of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein identical numerals indicate the same elements throughout the figures, FIG. 1 schematically depicts an insulator 10 which is suspended from a cross-arm 11 of a utility pole 12 and which supports an electrical conductor 13. The insulator 10 is a known suspension-type insulator having a generally cylindrical exterior shape with an upper end or grounded end fitting 14 and a lower end 16 connected to the electrical conductor 13, and is made from an electrically insulating (i.e., non-electrically-conductive) material. An exemplary sensor unit for accurately detecting and monitoring leakage currents according to an embodiment of the invention is mounted on the upper end 14 and shown generally at reference numeral 20.

In summary, the sensor unit 20 is an RF sensor which attaches to the insulator's grounded end 14, measures the leakage current flowing in the insulator 10, processes the current and assigns them to specific ranges. The sensor unit 20 then transmits the information wirelessly to either a local base station or a handheld device.

Referring to FIGS. 2 and 3, the sensor unit 20 includes a housing 21 for containing a leakage current sensor 22 and electronic module 70 (shown in FIG. 4), a door 23 hinged to the housing 21 to allow the sensor unit 20 to be positioned onto the insulator 10, and a clamping mechanism 24 for clamping the sensor unit 20 to the insulator 10. The housing 21 is a metal housing which encloses the sensor unit's functional components (described in detail below) and protects them from electromagnetic influences.

The door 23 is pivotally hinged to the housing 21 at hinge 26 to allow the door 23 to pivot away from the housing 21 to an open position and allow the sensor unit 20 to be installed on the insulator 10. Fastener 27 secures the door 23 to the housing 21 in a closed position once the sensor unit 20 has been installed. A hot stick adapter 28 is connected to the housing 21 and positioned next to fastener 27 to allow a user to use a hot stick for placement of the sensor unit 20 on the insulator 10. The hinge 26 is resistive in nature so that it moves freely, making it easier for the installer—especially under live line conditions.

Clamping mechanism 30 is secured to the housing 21 and includes first and second adjustable clamping jaws 31 and 32 connected to first and second pivotable arms 33 and 34, respectively. The jaws 31 and 32 are used to mechanically attach the sensor unit 20 to the grounded end 14 of the insulator 10 and are adjustable to account for different insulator end fitting diameters. As shown in FIG. 2, the jaws 31 and 32 include apertures 36 for receiving fasteners therethrough and to allow for adjustment of the jaws 31 and 32 with respect to arms 33 and 34. The jaws 31 and 32 are adjusted by removing fasteners 37, moving the jaws 31 and 32 relative to arms 33 and 34 until a desired aperture 36 (representative of a specific diameter) aligns itself with apertures in the arms 33 and 34. The fasteners 37 are then re-installed through the apertures in the arms and apertures 36 to secure the jaws 31 and 32 in position. As illustrated, the jaws 31 and 32 have a V or U shaped profile for mating engagement with the grounded end 14 of the insulator 10.

The jaws 31 and 32 are moved between opened and closed positions by a bolt mechanism 40 connected to threaded collars 41 and 42 disposed at an end of the arms 33 and 34, respectively. The bolt mechanism 40 includes a pair of opposing bolts 43 and 44 coupled together at their ends by a coupling 46. Each of the bolts 43, 44 extend through a respective one of the threaded collars 41, 42. As the bolts 43 and 44 are turned, the threaded collars 41 and 42 move along the length of the bolts 43 and 44, which in turn causes the arms 33 and 34 to move about pivots 47 and 48 such that they move the jaws 31 and 32 between opened and closed positions. The key feature of the bolt mechanism 40 is that it allows a user to turn either one of the bolts 43, 44 to open and close the jaws 31 and 32, thereby eliminating any issues with the user being on a specific side of the sensor unit 20. When installing the sensor unit 20 onto the grounded end fitting 14, the door 23 and jaws 31, 32 are moved to the open position for receiving the end fitting 14. The jaws 31, 32 are then moved to the closed position to clamp onto the end fitting 14. With the sensor unit 20 firmly clamped to the end fitting 14, the door 23 is then moved to the closed position and secured in place by fastener 27.

Referring to FIG. 4, the leakage current sensor 22 includes a toroidal current transformer 50 with a frequency response from <5 Hz to >100 kHz and a sensitivity of lower than 10 mA. The transformer 50 is formed by a leakage current toroidal ferrite core 51 and winding 52 wound around the core 51 with multiple turns. The core 51 is formed of two halves 51A and 51B so that it can surround the grounded end 14 of insulator 10. When the two halves 51A and 51B of the core 51 are put together, the voltage output from the windings 52 is proportional to the current flowing through the middle of the core 51.

A sheet metal housing 53 (steel which is ferromagnetic for magnetic field shielding) surrounds the core 51. As shown, the housing 53 is also formed by two halves 53A and 53B to surround respective core halves 51A and 51 B. The metal housing 53 is split along an inside with a metal slot 54, See FIG. 5, so that the core 51 can still couple with a magnetic field from current flowing in an insulator's metal end fitting. This enables the core 51 to be shielded from stray magnetic fields which are not due to current flowing through an insulator end fitting but still measure the currents flowing through the insulator metal end fitting. This is very important as there are large magnetic fields due to the currents flowing in the conductors which are energized at ends of the insulators.

As shown in FIG. 5, core 51B and housing 53B are installed in the door 23 so that the sensor unit 20 can be installed in the field on insulators without removing them from service. The core 51B and housing 53B are connected to a plate 56 which is connected to the door 23. The plate 56 is moveable relative to the door 23 and is biased towards the housing 21 by springs 57 and 58 to ensure that mating surfaces 59 and 60 of cores 51A and 51B are properly mated together. The mating surfaces 59 and 60 of the cores 51A and 51B are machined with key patterns so that alignment is ensured between the two cores. Alignment is vital so that the unit consistently measures the leakage currents flowing through the end fitting of the insulator, and it reduces the influence of stray magnetic fields.

Referring again to FIG. 4, an output 61 from the sensor 22 operably connects the sensor 22 to the electronics module 70. The electronics module 70 is imbedded in potting compound to protect it from both environmental and electromagnetic influences and includes analog, digital, and radio frequency (RF) electronics which function to receive, process, and store signals from the sensor 22, to receive external commands, and to transmit data to an external source. The electronics module 70 may include, for example, a printed circuit board incorporating analog, digital and/or radio-frequency (“RF”) electronic components or may incorporate discrete components and/or one or more microprocessors.

In addition to the electronics module 70, the housing 21 includes an electric power source for the electronics module 70, such as the illustrated batteries 72. The housing 21 also includes one or more RF antennas 63 which protrude from the exterior of the housing 21 and are used to transmit signals generated by the electronics module 70 to a remote receiver (not shown), and/or to receive RF signals from a remote receiver (not shown). The sensor unit 20 incorporates a communication system that may be based on the IEEE 805.15.4 architecture. The communication protocol is customized to allow two-way communications.

In the illustrated example, one or more magnetically-operated switches 71 are mounted inside the housing 21 and coupled to the electronics module 70. The switches 71 may be tripped by placing a magnet in the near vicinity of the switch 71 on the outside of the housing 21. In the illustrated example, the sensor unit 20 may include a power switch which toggles the sensor unit 20 between the on and off state, and a reset switch which signals the sensor unit 20 to erase any stored data.

The electronics module 70 may include a temperature sensor, in order to assist in assessing condensation conditions. The electronics module 70 may also include a 3D accelerometer, in order to assess whether the insulator or structure is experiencing vibration issues.

The operation of the electronics module 70 and the sensor unit 20 will now be described with reference to the block diagram in FIG. 6. In block 200, the electronics module 70 uses a peak detect circuit of a known type to measure and hold a voltage signal from the sensor assembly described above. The voltage signal is proportional to the highest leakage current measured in a predetermined first time interval, e.g. 60 seconds. This peak detect circuit is reset at the first interval, e.g. 60 seconds, based on a digital signal from the microprocessor. At block 202, an Analog to Digital (A/D) converter (which may be part of a microprocessor of the electronics module 70) measures the value from the peak detect circuit, repeating as the first interval. At block 204, the microprocessor evaluates the digital value and assigns the value to membership in a category or “bin”. The bin represents a range in which the measured value lies. For example, there may be six numbered bins which account for different leakage current magnitudes. Examples of two different settings for the leakage current threshold for the bins are listed in Table 1 below, in which “regular” indicates a classification that is relatively less sensitive to leakage currents and “sensitive” indicates a classification that is relatively more sensitive to leakage currents.

TABLE 1 CURRENT RANGE, CURRENT RANGE, BIN REGULAR SENSITIVE NUMBER (PEAK mA) (PEAK mA) 1  0-10 0-1 2 10-20 1-2 3 20-50 2-5 4  50-200  5-20 5 200-500 20-50 6 500+ 50+

The electronics module 70 maintains a counter for each of the bins. When the digital value is assigned to a bin, the counter for that bin is incremented (see block 206). At block 208, the number of counts in each bin is then transmitted using RF to a remote receiver. The transmission repeats at a second interval which is preferably shorter than the first interval described above.

The sensor unit 20 only stores the statistical parameters (i.e. the bin counts) of the leakage current peaks that occur. No other leakage current parameters need be recorded. Using the communications system described above, a remote user can reset the bins or change the ranges of the bins remotely.

The sensor unit 20 also keeps track of the time since the last reset. This limits the data message sent by the device to the bare minimum. It also limits the internal circuit complexity and power consumption for the device. Based on the battery characteristic and low power consumption of the sensor unit 20, battery life is estimated at more than 10 years.

The sensor units 20 can be employed in different modes. For example, when installed on transmission lines the sensor units 20 may be polled a only few times per year when line maintenance crews do inspections or maintenance, for example using handheld receivers (not shown).

Alternatively, in substations or on specific transmission line structures a more sophisticated approach may be followed. A dedicated base station system (not shown) installed at the substation/structure would poll the nearby sensor units 20 at a short time interval. This base station stores leakage current data together with weather parameters from attached sensors. The data is then transmitted from the base station using a variety of methods including the use of GPRS modems or connecting to a utility data management system. The data is stored and processed on a remote server. Alarms can be generated based on algorithms and data can be viewed using visualization tools.

If the leakage current characteristics of the insulators are known, alarms can be generated automatically based on preset leakage current alarm levels. Warnings or alarms can be raised to initiate insulator maintenance (e.g. washing) if certain pre-set leakage current thresholds are exceeded. Leakage current information can also be used select appropriate mitigation measures.

It should be noted that the sensor units 20 are connected between the insulator and the grounding system. With this configuration the insulator leakage current is directly shunted to ground and it will therefore not pass through the wood cross arm. The leakage currents measured can however be used to raise warnings that conditions and insulator contamination levels are sufficiently high to cause wood pole fires.

The sensor unit 20 described above has several advantages. The sensor units 20 are suitable for wide spread deployment which makes them practical for installations on overhead lines and substations. Some of the specific advantages of the sensor are low cost; absence of wiring to either power the sensor unit 20 or communicate with the sensor 20, leading to improved reliability compared to wired units; the ability to quickly deploy many sensor units 20; on-board processing of data, providing a user with processed information on which he can make a decision; and a combination of analog and digital electronics, ensuring that no current pulses are ever missed, as compared to previous technologies that used digital measurement only.

The foregoing has described a sensor apparatus for detecting and monitoring leakage current in suspension type insulators. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation. 

What is claimed is:
 1. A sensor apparatus for detecting leakage current in a suspension-type insulator of an electrical power system, the apparatus comprising: a housing having a first half of a leakage current sensor contained therein; a door pivotally connected to the housing by a hinge, and having a second half of a leakage current sensor contained therein, the door pivotable about the hinge between an open position in which the first and second halves of the leakage current sensor are separate from each other, and a closed position in which mating surfaces of the first and second halves of the leakage current sensor join together to define a closed perimeter and; a clamping mechanism connected to the housing comprising a first jaw and a second jaw, wherein the jaws are moveable between an open position and a closed position.
 2. The apparatus according to claim 1, wherein: the first jaw is supported by a first arm; the second jaw is supported by a second arm; a first threaded collar is connected to the first arm; a second threaded collar is connected to the second arm; and a bolt mechanism is disposed in threaded engagement with the first and second arms.
 3. The apparatus according to claim 2, wherein the bolt mechanism includes first and second opposing bolts coupled together at their ends by a coupling.
 4. The apparatus according to claim 2, wherein the first and second jaws include a plurality of apertures for receiving a fastener therethrough.
 5. The apparatus according to claim 1, further including a hot stick adapter mounted to the housing.
 6. The apparatus according to claim 1, further including a fastener for securing the door in the closed position.
 7. The apparatus according to claim 1, further including an electronics module configured to receive a signal from the leakage current sensor and a communications system operable to wirelessly transmit the signal to an external receiver.
 8. The apparatus according to claim 1, wherein the leakage current sensor is operable to generate an analog signal proportional to a received leakage current.
 9. The apparatus according to claim 1, wherein the leakage current sensor includes a current transformer having: a ferrite core having windings wrapped therearound; and a metal housing around the ferrite core and windings to shield the current transformer from magnetic fields, wherein the metal housing includes a slot extending along an inner surface thereof.
 10. A method of detecting leakage current in a suspension-type insulator of an electrical power system, the method comprising: installing a grounded end fitting of the insulator in the apparatus of claim 7; conducting a leakage current from a grounded end fitting of the insulator through a current transformer of the leakage current sensor; generating an analog signal proportional to the leakage current flowing through the current transformer; using the electronics module to receive the analog signal and convert the analog signal to a digital value; and using the communications system to wirelessly transmit the digital value to an external receiver.
 11. The method according to claim 10, further including the step of using the electronics module to assign the digital value to one of a plurality of bins based on a magnitude of the digital value, wherein each of the plurality of bins represents a predetermined range of magnitudes of leakage current.
 12. The method according to claim 11, further including the step of incrementing the assigned bin each time a digital value is assigned to the bin.
 13. The method according to claim 10, further including the step of using the electronics module to detect a peak value of the analog signal at a first interval.
 14. The method according to claim 13, further including the step of using the electronics module to convert the peak value to the digital value.
 15. The method according to claim 10, wherein the step of installing the grounded end fitting includes the steps of: moving the door of the apparatus to an open position; moving the jaws to an open position; receiving the grounded end fitting between the jaws and moving the jaws to the closed position, such that the jaws clamp onto the grounded end fitting and secure the sensor apparatus to the insulator; and moving the door to the closed position and securing the door in the closed position. 